This invention relates to a fuel supply apparatus and method for internal combustion engines such as gasoline engines and more particularly to a fuel supply apparatus which adjusts an air-fuel ratio in accordance with decreases in the temperature of intake air to the engine due to heat of vaporization of fuel.
For example, a fixed venturi carburetor used extensively as a fuel supply apparatus for a gasoline engine is designed to control intake air flow by means of a butterfly throttle valve provided within an intake air passage to the engine and associated with an accelerator pedal, to measure the quantity of intake air in terms of the vacuum pressure in the fixed venturi, and to adjust the difference between pressures upstream and downstream of a fuel jet which meters fuel in accordance with the vacuum pressure differential in order to approach a desired air-fuel ratio. An SU carburetor typical of variable-venturi carburetors controls intake air flow by means of a butterfly throttle valve, measures the controlled intake air flow in terms of the equilibrium position of a suction piston which slides so as to hold the velocity of intake air in the variable venturi constant, and adjusts the flow cross-section of a fuel nozzle in accordance with the equilibrium position, thereby metering fuel.
A recently extensively used electronically controlled fuel injection apparatus measures intake air flow by means of an air flowmeter or a pressure sensor, calculates a required quantity of intake air in accordance with the measured quantity of intake air and controls fuel flow in accordance with the time interval of opening of a fuel injector.
In summary, in each of the above-mentioned prior art fuel supply apparatus, the actual air-fuel ratio of the air-fuel mixture is not directly measured, nor is the actual air-fuel ratio of air-fuel mixture derived after the corresponding measurements of intake air flow and fuel flow are fed back. Thus, the air-fuel ratio is liable to be influenced by various factors such as the accuracy of machining of the needle valve, so that it is difficult to realize air-fuel ratio control with the high precision necessary for exhaust emission control and fuel economy.
Recently, an air-fuel ratio control system has been put to practice in internal combustion engines using a three-way catalyst which senses the quantity of oxygen remaining in the exhaust emissions to recognize the magnitude of the air-fuel ratio, thereby controlling fuel flow in the carburetor by feedback. This method serves only to hold the air-fuel ratio at a constant, stoichiometric value and determines the air-fuel ratio solely on the basis of the exhaust emissions, so that this system cannot be used in conjunction with a secondary air introduction device (See "SERVICE WEEKLY No. 472" (page 56) published October 1982 by NISSAN MOTOR COMPANY, LTD.)